The invention relates to a circuit for blocking a semiconductor switching device on overcurrent, the semiconductor switching device having at least one continuously driven semiconductor switch, which circuit comprises a driver circuit having a driver stage for each semiconductor switch, a control pulse generator for producing control pulses, which are fed in operation to a control input of the semiconductor switching device via the driver circuit, and a monitoring device, which measures the current flowing through the semiconductor switching device and which, when an overcurrent occurs, generates a fault signal, which initiates blocking of the semiconductor switching device.
The semiconductor switching device is generally an inverter having several power switching transistors in the form of semiconductor switches.
In a known circuit of that type (EP 0 521 260 B1), free-wheeling diodes are connected anti-parallel to each semiconductor switch in order to avoid, at the semiconductor switches, overvoltages that are caused by inductive resistors, such as choke coils, inductive loads or lead inductances, in the circuitry of the semiconductor switches when a semiconductor switch is switched off (blocked) in normal operation. When an overcurrent, for example a short-circuit current, flows through the semiconductor switches, it is, however, possible for even higher overvoltages to occur. The known circuit should reduce those overvoltages by blocking one of the series-connected semiconductor switches simultaneously carrying an overcurrent, without increasing the amount of circuitry involved by using capacitors. Notwithstanding, free-wheeling diodes are still required. Even when those are provided, when a semiconductor switch carrying a very high overcurrent, such as a short-circuit current, is being blocked, in the circuit of which semiconductor switch there is a high inductive reactance, a very high overvoltage can still occur at the blocked semiconductor switch.
The invention is based on the problem of providing a circuit of the kind mentioned at the beginning that allows, with little outlay, a further reduction in an overvoltage at the semiconductor switching device when that is being blocked because of an overcurrent.
According to the invention, that is achieved by means of the fact that the operating voltage of the driver circuit is arranged to be switched over briefly by the fault signal to a lower, interim value corresponding to a lower current through the semiconductor switching device and then, within the maximum permissible duration for loading the semiconductor switching device with an overcurrent, to be switched off completely.
In this solution, therefore, the overcurrent is reduced to zero in stages. For each switching-off stage, the amount by which the current flowing through the semiconductor switching device decreases is, therefore, also smaller. Consequently, the rate of change (di/dt) of the current is correspondingly lower for each switching-off stage, as is, therefore, the voltage induced in the inductive reactance in the circuit of the semiconductor switching device by the change in the current (Ldi/dt). Because the induced voltage is added to the operating voltage of the semiconductor switching device when the semiconductor switching device is being blocked, the total overvoltage at the semiconductor switching device when the blocking occurs is also lower. The semiconductor switching device is, therefore, not unduly loaded and does not require additional circuitry to reduce overvoltage when blocking occurs.
Provision is preferably made such that, for several semiconductor switches jointly supplied from one operating voltage source, the current flowing through the semiconductor switches is measured in a supply line common to all th e semiconductor switches by the monitoring device, a single measuring device in the monitoring device being sufficient for all the semiconductor switches.
Provision can then be made such that, for several semiconductor switches, the driver stages thereof are all supplied from a common operating voltage source, which is galvanically isolated from the driver stages and which, as a function of the fault signal, is arranged to be switched over to the interim value and switched off. In that arrangement, there is no need, when an overcurrent occurs in a semiconductor switch, to determine which semiconductor switch is affected. There is, accordingly, less outlay on resources in the monitoring device.
An advantageous practical form of the circuit can consist in that the operating voltage source of the driver circuit is a direct-current voltage source, which is connected, via a chopper controlled by a pulsed switching signal and a transformer having a secondary winding for each driver stage and via a rectifying circuit connected to the secondary winding, to a (respective) driver stage and the switching signal that controls the chopper is frequency- or pulse-length-modulated as a function of the fault signal. In that arrangement, the reduction in the operating voltage of the driver circuit when an overcurrent occurs is achieved by conversion of the operating voltage into a pulsed voltage and subsequent frequency- or pulse-length-modulation of the pulsed voltage.
The control pulses of the control pulse generator can be fed to a control input of the driver circuit in a customary manner.
Provision is preferably made such that the control pulses and the operating voltage for each driver stage are transmitted by means of a high-frequency carrier signal of an oscillator that is common to all the driver stages, via the same galvanic isolation stage. In that arrangement, galvanic isolation between the switching device, which is optionally operated with high voltage, and the low-voltage-operated switching circuits controlling the driver stage(s) thereof is possible with little outlay on isolation stages.
A simple practical form can consist in combining the control pulses of the control pulse generator (when using frequency- or pulse-length-modulation of a driver circuit direct-current operating voltage converted into a pulsed intermediate circuit voltage by means of the chopper) with the switching signal controlling the chopper by means of an AND gate, in order to transmit the control pulses and the operating voltage to the driver stages with galvanic isolation.
A further alternative arrangement of the operating voltage source of the driver circuit can consist in that it has an output for a high, normal value and an output for the low, interim value, one of which outputs can be selected for the supply of the operating voltage as a function of the fault signal.
In that arrangement, the outputs can be connected by an OR gate.
Provision can then be made such that one output is connected, via a diode, to one end of the switching path of a controllable switch, the other output is connected, via a diode, to the other end of the switching path and to the driver circuit, and the switch is arranged to be switched as a function of the fault signal.